When investigators perform in vivo MR imaging of thoracic organs, movement of tissues is caused by the cycle of muscular contractions of the atria and ventricles of the heart, and by the cycle of respiration created by movement of the diaphragm, even while the subject is nominally in a resting condition. These two sources of movement can cause motion artifacts in MR images. Previous work provided methodologies to reduce or eliminate artifacts in image acquisition caused by cardiac and respiratory movements. See for example, Wang Y, Rossman P J, Grimm R C, Riederer S J, Ehman R L. Navigator-echo-based real-time respiratory gating and triggering for reduction of respiration effects in Coronary Artery Disease 437 three-dimensional coronary MR angiography. Radiology 1996; 198:55-60; Danias P G, McConnell M V, Khasgiwala V C, Chuang M L, Edelman R R, Manning W J. Prospective navigator correction of image position for coronary MR angiography. Radiology 1997;203:733-736; McConnell M V, Khasgiwala V C, Savord B J et al. Prospective adaptive navigator correction for breath-hold MR coronary angiography. Magn Reson Med 1997;37:148-152; McConnell M V, Khasgiwala V C, Savord B J, Chen M H, Chuang M L, Edelman R R, Manning W J. Comparison of respiratory suppression methods and navigator locations for MR coronary angiography. AJR Am J Roentgenol 1997;168:1369-1375; and Kotys M S, Herzka D A, Vonken E J, Ohayon J, Heroux J, Gharib A M, Stuber M, Pettigrew R I. Profile order and time-dependent artifacts in contrast-enhanced coronary MR angiography at 3T: origin and prevention. Magn Reson Med. 2009 August; 62(2):292-9.
Cardiac-Gating Technique
The effect of cardiac movement can be reduced by using ECG data collected from the subject to trigger MR image acquisition. By using ECG data to trigger image acquisition, acquisition can be acquired only when the heart is in diastole. This technique is called cardiac triggering, and serves to reduce the cardiac motion artifacts that would otherwise be observed.
Diaphragm-Gating Technique
Respiratory movement is controlled for with the so-called navigator technique. In the navigator technique, the position of the top of the diaphragm is monitored with a navigator “pencil beam” radio-frequency (RF) pulse that measures the location of the dome of the right side of the diaphragm. Using this information on diaphragm position, MR image acquisition is only performed when the diaphragm is in a predetermined window of location. Specifically, in the conventional navigator gating method, a free navigator is performed to get the most stable and consistent diaphragm position. The acquired data is only accepted when the diaphragm is within the navigator window. If there is motion outside of the navigator window, the scanner software rejects the acquired data. Additional data samples to make up for the rejected data are then acquired until the data are sampled when the diaphragm is in position within the navigator window.
When the ECG-triggering and navigator-gating techniques are combined, they can control for the movements of the cardiac and respiratory cycles. However, they do not control for another source of artifact, namely, that resulting from variation in spin condition of nuclei in the tissues being imaged.
When an RF signal acquisition step (also referred to as a “shot” or as a “normal shot”) is conducted, it affects the condition of the spin of nuclei in the tissues being imaged, causing a deviation from the relaxed position. A “shot” can be a series of RF pulses in the case of a fast imaging sequence. If the spin of nuclei is not fully relaxed when the next shot is conducted, an artifact is created. For example, if an M2D and single-shot turbo field echo (TFE) pulse sequence, combined with the normal ECG-triggering and navigator-gating techniques, is used to acquire sagittal heart images, the intensity of different slices is different, which results in a severe “banding artifact” once the transverse or coronal images are reconstructed from the sagittal images. This banding artifact impairs the ability of clinicians to accurate diagnose heart disease. In the M2D and single-shot turbo field echo (TFE) pulse sequence method, multiple image slices are excited one after the other and each slice is acquired within one heart beat.
Also known in the prior art is Pines et al., U.S. Pat. No. 7,061,237, issued Jun. 13, 2006, which is said to disclose an apparatus and method for remote NMR/MRI spectroscopy having an encoding coil with a sample chamber, a supply of signal carriers, preferably hyperpolarized xenon and a detector allowing the spatial and temporal separation of signal preparation and signal detection steps. This separation allows the physical conditions and methods of the encoding and detection steps to be optimized independently. The encoding of the carrier molecules may take place in a high or a low magnetic field and conventional NMR pulse sequences can be split between encoding and detection steps. In one embodiment, the detector is a high magnetic field NMR apparatus. In another embodiment, the detector is a superconducting quantum interference device. A further embodiment uses optical detection of Rb—Xe spin exchange. Another embodiment uses an optical magnetometer using non-linear Faraday rotation. Concentration of the signal carriers in the detector can greatly improve the signal-to-noise ratio.
There is a need for systems and methods for providing improved in vivo magnetic resonance images of subjects.